Permalinks and other items can be dropped when importing mails

The database design includes some fields in the mbox index entry which are not directly derived from the email.

These include:
- permalinks
- deleted flag
- previous, top, thread
- _notes, _archived_at

These fields will be lost if an entry is added to the index with the same id.
This may occur when importing historic emails, e.g. to backfill missing messages.

It's not unknown for the same message to be delivered twice to the same list.
Such deliveries normally occur within a short space of time.
It's not yet clear whether significant mbox fields can be lost in this way, but _notes and _archived_at will be affected, as will the dbid entry which points to the source index. This will generally result in an orphan source entry.

Start by tracing the mbox index import path for historic emails and inspect how entries with an existing id are handled. Check preservation of permalinks, deleted, threading fields, _notes, _archived_at, and the dbid source-index link; done means duplicate or backfilled imports do not orphan or overwrite these fields.

Written by the indexing model from the issue text.